1. Field of the Invention
The present invention relates to an electrical fuse and a semiconductor device.
2. Description of the Related Art
Conventionally, there has been known a technology of cutting a fuse mounted in a semiconductor device, to thereby perform various processes including adjustment of a resistance value of the semiconductor device and detachment of a defective element to be replaced with a normal element.
As for a method involving cutting the fuse, there are known a method of irradiating a part of the fuse with a laser to cut the fuse and a method of causing a current to flow through the fuse, to thereby cut the fuse.
JP 2004-304002 A discloses the following technology. In a semiconductor device which includes a fuse including a fuse main body and two pads coupled to each other via the fuse main body, and two conductive layers respectively coupled to the two pads, a length of the fuse main body is defined so that, when the fuse is blown through application of an electrical stress between the two conductive layers, the blown portion of the fuse is positioned within the fuse main body, which is separated from an area overlapping with the conductive layer. This technology is aimed to enable more reliable fuse blow out.
Further, as an example of an electrical fuse to be cut by causing a current to flow therethrough, there is known an electrical fuse which employs a phenomenon in which its constituent material migrates due to electromigration (see, for example, JP 2005-39220 A).
The present inventor has recognized as follows. As described in JP 2005-39220 A, in the case of cutting the fuse employing a phenomenon in which its constituent material migrates due to electromigration, there is a fear that, when a semiconductor device is subjected to heat treatment after the fuse has been cut, the constituent material may migrate again due to electromigration and thus the fuse may be reconnected at the cutting portion. When such reconnection occurs, even after the cutting-target electrical fuse has been cut, accurate results may not be obtained through detection of whether or not the electrical fuse has been cut.
FIGS. 7A to 7C illustrate an example of an electrical fuse using copper as a constituent material.
FIG. 7A illustrates a structure of an electrical fuse 10 before cutting. The electrical fuse 10 includes terminals 20 and 22 and an interconnect to be cut 12 provided between the terminals 20 and 22. The interconnect to be cut 12 has a width smaller than widths of the terminals 20 and 22. When a current is caused to flow in a direction from the terminal 20 toward the terminal 22 of the electrical fuse 10, electrons migrate within the interconnect to be cut 12 in a direction from the terminal 22 toward the terminal 20 as illustrated in FIG. 7B. Along with the migration of electrons, copper forming the electrical fuse 10 migrates due to electromigration, whereby a void 30 is generated within the interconnect to be cut 12 having the smaller width. In some cases, however, the void 30 formed within the interconnect to be cut 12 which is cut utilizing electromigration is not large enough to extend over the width of the interconnect to be cut 12. As a result, there is a case where the electrical fuse 10 is not cut completely or a case where the electrical fuse 10 is reconnected due to electromigration when heat treatment is performed after the cutting, as described above.
The possibility that the reconnection as described above occurs is not so strong as to incur a problem as long as a semiconductor device is used in a normal operation. However, when significantly high reliability is required for a semiconductor device, or when a semiconductor device is used under severe conditions, it is necessary to further enhance characteristics of maintaining a cut state of the electrical fuse after cutting.
Incidentally, JP 2001-68475 A discloses a process of allowing, in an interconnect containing copper or a copper alloy layer, a majority of copper or copper alloy crystal grains to form twin crystals. Such twin crystals of the copper-based film form a coherent twin-crystal boundary. It is described in JP 2001-68475 A that an electromigration rate is low in the coherent twin-crystal boundary, and accordingly the two crystal grains forming the twin-crystal boundary may be substantially regarded as a single large crystal grain. The inventor(s) of the present invention have found that the reconnection of the electrical fuse after the cutting as described above may be prevented by using such twin crystals in the electrical fuse, and arrived at the present invention.